NANOG and cancer: The major cancer stem cell specific pathways, including phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR), maternal embryonic leucine zipper kinase (MELK), NOTCH1, and Wnt/β-catenin, and genes (including CD133, CD24, CD44, OCT4, SOX2, NANOG and ALDH1A1), maintain cancer stem cell properties [60].